Grinding machines are often used to abrade material from a workpiece, such as a forging or casting, or to impart a finish to the workpiece. One such grinding machine is known as a backstand grinder, and it includes a contact wheel and at least one idler wheel, which support an endless coated abrasive belt. The relative positions of the contact wheel and the idler wheel are adjustable to maintain adequate tension in the abrasive belt. The workpiece may then be pushed against an exposed abrasive face of the abrasive belt, either manually or by a machine, in the area where the belt is supported by the contact wheel, to abrade material from the contacted surface of the workpiece. The area where the workpiece contacts the abrasive face of the belt will be referred to herein as the abrading interface.
Contact wheels may be constructed in one of several manners. For example, the entire contact wheel may be constructed from a material such as rubber or steel. Alternately, the contact wheel may include a rigid inner hub (constructed of metal, for example) surrounded by a resilient tread (constructed of rubber, for example).
The surface characteristics of a contact wheel have been shown to impact the cut rate (i.e. the rate of material removal from the workpiece by the abrasive belt) and the resultant surface finish of the workpiece. A plain-faced contact wheel, which has a generally cylindrical continuous outer periphery, is typically used for very fine polishing or burnishing, depending on the durometer of the wheel. Plain-faced contact wheels provide the lowest effective pressure at the interface between the workpiece and the abrasive belt. However, it may be desirable to increase the effective pressure at the abrading interface, which led to the development of the serrated, or grooved contact wheel.
Serrated contact wheels include a plurality of grooves formed in and extending across the peripheral face of the contact portion of the wheel. These grooves result in a contact wheel having alternate lands and grooves in the face of the wheel, which increase the effective pressure at the abrading interface. Because serrated contact wheels produce higher effective pressures at the abrading interface, they produce a higher cut rate. Factors such as the ratio of groove width to land width, the depth of the groove, the shape of the land, and the hardness of the wheel each affect the cut rate and the performance of the coated abrasive belt. Thus, serrated contact wheels are particularly suitable for grinding operations that require a relatively large amount of material removal.
Serrated contact wheels, however, may engender certain difficulties that make such wheels undesirable. For example, constant flexing of the land portions of the contact wheel tends to cause fatigue, and the life cycle of the contact wheel may therefore be diminished. Furthermore, abraded material may tend to build up on the hub of the contact wheel, which can cause the wheel to become imbalanced and to abrade unevenly--a problem that is also germane to plain-faced contact wheels. Most importantly, when an operator abrades a workpiece at the edge of the contact wheel, higher effective pressures can occur, which can result in damage to or destruction of the outer edge of the abrasive belt. The abrasive belt tends to become "shelled," meaning that the abrasive particles cease to be bonded to the belt, and may be stripped from the backing during abrading. Given that the land areas at the edges of the contact wheel are particularly susceptible to flexing, and tend to become fatigued relatively quickly, the serrated contact wheel may be inadequate for some applications.
A third type of contact wheel that is thought to overcome the problems described above is a hybrid contact wheel. As shown with respect to the present invention in FIG. 2, a hybrid contact wheel includes circumferentially spaced grooves in the peripheral face of the contact portion of the wheel. These grooves are spaced from each side of the contact wheel, which provides annular land surfaces at each outer edge of the wheel. Thus, the contact wheel in essence includes serrated portions and plain-faced portions. The hybrid contact wheel decreases flexing of the land portions, particularly near the outer edges of the wheel, which results in longer product life. The edges of the belt, due to the lower pressure superjacent the annular, plain-faced land surfaces, last longer and are more resistant to shelling. Finally, the cut rate proximate the center of the belt is greater than a serrated contact wheel, perhaps due to the increased support provided by the annular support portions. Thus the hybrid contact wheel solves many of the problems displayed by the plain-faced and serrated contact wheels.
The hybrid contact wheel exhibits at least two undesirable side effects. First, abraded material may accumulate on the hub of the contact wheel, potentially resulting in wheel imbalance and uneven abrading. Second, and more critically, hybrid contact wheels typically generate unacceptably high levels of noise during use. For example, noise measurements proximate the contact wheel (i.e. 8 inches away) may approach 110 dBA (see infra, Example One). The threshold of pain in response to sound may, depending on the frequency, be between 110 and 130 dBA. Thus, prolonged exposure to the noise produced by a contact wheel, if not painful, may be irritating or uncomfortable to an operator who is working on or near the abrading apparatus. In addition to the potential for hearing difficulty, increased noise makes communication on the shop floor more difficult, with the concomitant risks of an accident or injury due to an inability to communicate effectively.
It is therefore desirable to provide a hybrid contact wheel that produces less noise, and is less susceptible to the build-up of abraded material on the hub, than known hybrid contact wheels.